The invention relates to an RF-input stage comprising a field effect transistor in a grounded source circuit configuration, the gate electrode of which is connected to an aerial input via a frequency-dependent coupling network and to ground via a resistor. Such a stage may be used in T.V.-receivers.
Such an RF-input stage is disclosed in U.S. Pat. No. 3,482,167.
The frequency-dependent coupling network of the prior art RF-input stage has inter alia for its function to realize an impedance transformation between the low impedance (approximately 75.OMEGA.) aerial input and the high impedance (approximately 3k.OMEGA.) input of the field effect transistor. Such a transformation of the impedance can only be realized in a satisfactory way for a comparatively narrow frequency range, for example to the order of 2 to 3 TV-channels. In order to enable signal processing of the TV-channel within at least one TV frequency band the known, comparatively narrow-band filter element is of a tunable construction. The frequency-dependent coupling network then also serves to provide a given channel selection.
In order to somewhat equalize the bandpass characteristic, the frequency-dependent coupling network is loaded with the resistance. At small resistance values, the signal loss through the resistor may become particularly large, so that the signal-to-noise ratio of the RF-input stage decreases. Therefore, in actual practice this resistance is selected rather high in value, generally higher than the input impedance of the field effect transistor.
As the prior art coupling network realizes a comparatively large impedance transformation, it may happen that noise signals at the aerial input result in an impermissible large cross-modulation, even at a very small noise amplitude.
In addition, the tuning of the frequency-dependent coupling network must be accurately locked to the tuning of the local oscillator of the TV receiver in which the known RF-input stage is used. Consequently, it is rather costly to realize the known RF-input stage.